The present invention is related to power system analysis and, more particularly, to systems which can automatically identify the power system type and likely errors of wiring and connections in the power system.
One of the biggest problems in measuring aspects of electrical circuits is simply the avoidance of errors. For example, errors in the wiring of an electrical circuit connected to a power system are not obvious, but they can drastically affect the resulting measurements of the circuit. Errors in connecting measurement sensors to the circuit are quite common and often result in unusable measurements or misinformation.
Traditionally, such errors are minimized by the knowledge, training and experience of a person, a professional who makes the proper test connections to the circuit and then evaluates the preliminary measurements to determine if the wiring, connections, setup, and operating modes are correct. Based on these preliminary measurements, the professional then makes the necessary corrections. The manufacturers of test equipment have also attempted to minimize errors by: educating the professional with sufficient information so that he or she can make informed decisions on existence of errors and what to do to rectify the errors; requiring the professional to specify the type of power system under analysis and then, based on the specification, possibly indicating to the professional some limited level of error detection; or constraining the professional to hook up the test equipment to only a small number of basic power system types.
Heretofore, a major problem, if not the major problem, with error detection and interpretation has been that there is often more than one possible explanation for a given set of input signals to the test equipment. For instance, if the voltage from A phase to neutral (Van) appears to be 120V, the B phase voltage to neutral (Vbn) appears to be 120V, and the C phase voltage to neutral (Vcn) appears to be 0V, there are several likely explanations:
1) the user is measuring a 2-phase circuit and should measure the phase-to-phase voltage, Vab, of 240V;
2) the user is measuring 2 different single phase circuits and all setups are correct;
3) the user is measuring 1 single phase circuit and the ground lead is not connected;
4) the user is measuring a 3-phase Wye circuit and the C phase is not connected;
5) the user is measuring a 3-phase Delta circuit and the C phase is not connected; or
6) the user is measuring a 3-phase, 4-wire Delta circuit and the C phase has not been connected.
Furthermore, complexity is multiplied when current inputs to the test equipment are also considered. Since a the phase of a current can lead its associated voltage by up to 90 degrees or can lag the associated voltage by up to 90 degrees, if the current sensor is installed backwards, the current leads its voltage from 90 to 270 degrees. In a three-phase power system, each voltage phase is typically 120 degrees apart. Hence, if a current is seen to lag a voltage by 60 degrees, there are several possibilities:
1) the current is lagging its voltage by 60 degrees;
2) the current sensor is misconnected and it should be matched to the next voltage phase, which it leads by 60 degrees; or
3) the current sensor is installed backwards and is misconnected. It should be paired with the third phase, with which it is actually in phase.
Identification of a power system is further complicated by the fact that there are multiple “standard” power systems and multiple configurations of standard power systems. For instance, at 60 Hz, there are several standard three-phase, phase-phase systems, such as 120V, 208V, 240V, 480V, 600V, 4160V, 12470V, 13800V, etc. At 50 Hz and 400 Hz, there are several more standard three-phase, phase-phase systems. By adding consideration of a neutral, there are many more three-phase, phase-neutral systems, such as a 120V Wye system. And to complicate matters even more, there are two-phase systems, single-phase systems, DC systems, and the four-wire Delta system with its asymmetric voltage levels and phase angles between phases. All considered, there are dozens of unique standard power delivery systems in use in the world.
Another difficulty in identifying a power system is that most systems are not ideal. For example, in an ideal three-phase power system, all voltages are equal and nominally non-varying. The voltages have a specific frequency with a fixed phase relationship with respect to each other. A particular power delivery system might have 480 Vrms phase-to-phase at 60 Hz with 120 degrees between each of the phases and the current of each of the phases would be expected to lag its associated voltage by the same number of degrees as in the other phases. All currents would have essentially the same magnitude. But individual voltages typically vary from moment to moment, as do currents and phase angles between voltages and their associated currents. Even the frequency can vary in some systems. A non-ideal system must be recognized and a varying one must not be confused with a similar but different system. A judgment must be made whether the system is standard or non-standard, because there are non-standard systems and sometimes errors cause standard systems to operate outside of acceptable bounds.
To complicate matters even further, there may be errors in the connection of the measurement devices to the system or in the wiring of the system itself, as often happens with temporary measurements of a power system. In such cases, it can be very difficult to recognize a power system. When measuring power, it can be impossible to state with certainty what type of power system is present, whether the connections to the power system are correct, and what errors have been made in the wiring or connections to the power system. For instance, if three current sensors are rotated (connected by one phase off so that A phase is connected to B phase, B phase is connected to C phase, and C phase is connected to A phase), it could appear to be the same as if the sensors were connected to the correct phases, but were installed backwards. Resolution of such issues requires experienced judgment.
Therefore, a need exists for a system that can automatically determine the most likely power system that is connected to a circuit or system under test and the most likely connection or wiring errors present when taking the measurements. To address these issues and problems, the present invention combines methods of interpreting measurements, methods of analysis, and methods of using power system databases, and rules from an artificial intelligence knowledge base in order to make better judgments about the correctness of the wiring and connections, perhaps as well as or even better than a typical knowledgeable professional, and then to communicate the most likely errors (if any) to the user for a clear statement of likely remedial steps which are warranted. The present invention uses artificial intelligence applied to a knowledge base and interpretation of a set of measurements to determine the most likely power system that is connected and the most likely connection or wiring errors present.